Preheat oven to 400 degrees F. Meanwhile, cut the bulb of each spring onion into quarters.
On a rimmed baking sheet, toss all onions with oil and salt.
Roast, turning occasionally, until onions soften and develop a light char, 10 to 20 minutes.
(If the scallions cook faster than the leeks and spring onions, transfer the scallions to a plate and cover with foil until the other onions are ready.)